A number of resistive sheet or resistive layer antenna designs and systems exist within various industries for providing a partly conductive and at the same time optically transparent layer of material for multiple applications. An antenna feeding mechanism is associated with each of these antenna systems. The sheet resistivity and the light transparency of the resistive sheet are the key factors that determine the implementation of a resistive sheet antenna. In general, an antenna made of a resistive transparent sheet, such as Indium tin oxide, experience losses several orders of magnitude larger than an antenna made of a conductive material such as copper or silver. Therefore, antennas are primarily made of a conductive material, if possible. However, conductive materials are opaque to light. As a result, in certain applications requiring the use of a transparent antenna, a conductive material cannot be used.
In recent years, the demand for transparent antennas has increasingly grown for touchscreen, mobile platform, and automobile applications. In particular, the implementation of antennas, made of a transparent conductive layer, on the display window of a portable communication device have been addressed in the prior art, as described in U.S. Pat. No. 7,983,721 to Ding et al., the specification of which is incorporated herein by reference in its entirety. However, these efforts have faced certain challenges and limitations. Particularly, attempts made to provide an antenna design sufficiently transparent to light and at the same time capable of performing at radiation efficiency levels set up by industry standards have not been successful. A major challenge is that as the sheet resistivity of a resistive sheet decreases, making the resistive sheet more conductive, the optical transparency of the resistive sheet decreases. Likewise, as the sheet resistivity increases, the power dissipated as heat as a result of currents flowing on the resistive sheet increases too. Accordingly, the radiated power and the radiation efficiency of the resistive sheet are reduced, making it very challenging for resistive sheet antennas to meet radiation efficiency industry standards.
As a result, a compromise is required between two conflicting goals. Firstly, making the resistive sheet as conductive as possible, which means less transparent; and secondly, making the antenna more optically transparent, which means a more resistive sheet having a larger sheet resistivity. Current technology offers optically transparent resistive sheets having a sheet resistivity larger than 10 Ohms per square. However, for these values of sheet resistivity, standard design techniques used for antennas made of conductive materials notably fail.
The antenna feeding mechanism plays a crucial role in the overall radiation efficiency of the feeding-antenna system. Typically, standard feeding design techniques will enable the excitation of radiofrequency (RF) currents on the resistive sheet antenna at the expense of significant power losses. Two key reasons account for these losses: first, usually there are large concentrations of RF currents on the resistive sheet at the transitioning area between the transmission line used to feed the antenna and the resistive sheet antenna; and second, the non-uniform distribution of RF current densities excited on the resistive sheet. In general, the amount of power losses significantly increases as the sheet resistivity increases.
Moreover, in placing an antenna or a feeding mechanism close to conductive or resistive materials, electromagnetic coupling between the antenna and these materials also contributes to power losses that decrease the effective radiated power at a system level. In most touchscreen and mobile platform applications, the antenna-feeding system is surrounded by a number of conductive and resistive materials that must be considered, especially when designing an antenna using resistive sheets, to maximize the overall radiated power. Accordingly, manufacturers intending to use a resistive sheet on the touchscreen area as an antenna experience either an unacceptable reduction in radiation efficiency or an unacceptable performance of the touchscreen. This leads manufacturers to implementation of antenna systems that are costly, aesthetically unappealing, or more importantly, highly inefficient.
Previous efforts have been made to develop a method of improving the radiation efficiency of antennas made of transparent resistive sheet, as described in U.S. Pat. No. 7,233,296 to Song, et al., the specification of which is incorporated herein by reference in its entirety. However, this method is primarily aimed at determining values for current density over the surface of the resistive sheet to identify regions having concentrated flow of currents. Then the antenna efficiency is improved by increasing the conductivity in such areas of high current concentration.
The method described in the patent to Song et al., has also faced severe challenges and limitations. In particular, the resulting resistive layer will not be optically homogeneous. In other words, there will be areas of the resistive layer having darker spots resulting from the increased conductivity. Thus, although the resistive layers may meet optical transparency functional requirements, the resistive layer will not be aesthetically appealing. Furthermore, the manufacturing process used to provide different regions with different conductivity increases costs. Moreover, and more importantly, the areas of high-current concentration will vary depending on the type of application, the user operation, and the surrounding areas to the resistive sheet. Accordingly, small areas of higher conductivity on the resistive sheet may not cover a shift of the high-current spots. Alternatively, increasing the size of the areas of higher conductivity (darker areas) on the resistive sheet may further compromise the aesthetics and the optical transparency of the resistive sheet.
Furthermore, Bayram et al., as described in copending and co-owned U.S. patent application Ser. No. 14/252,975 titled “Antenna and Method for Optimizing the Design Thereof” (the specification of which is incorporated herein by reference in its entirety), have disclosed an approach for improving the radiation efficiency of a resistive sheet antenna, based on the topology design of the resistive sheet. In this approach, the radiation efficiency of the antenna is primarily increased by either reducing or preventing RF current “hot spots” and “pinch points” flowing on the resistive sheet. While this approach is effective in reducing or preventing high concentrations of RF currents, once they are flowing on the resistive sheet, a major limitation may result where the feeding mechanism is based on standard feeding designs. As a result, this approach is not able to prevent the non-uniform distribution of RF current densities and large concentrations of RF currents on the resistive sheet at the transitioning area between the transmission line used to feed the antenna and the resistive sheet antenna. Thus, even if the radiation efficiency of the antenna is improved, the power losses at the feeding transitioning area may result in an overall efficiency of the feeding-antenna system that is unacceptable to meet industry standards.
An RF current “hot spot” is characterized by a region of a material wherein a concentration of RF current is present having significantly larger current levels as compared to other regions having a more uniform current distribution and lower current levels. In particular, for a resistive sheet, a “hot spot” region dissipates a substantial amount of power as heat, significantly reducing the amount of radiated power.
Likewise, an RF current “pinch point” is characterized by a region of a material wherein the physical configuration of the material forces the RF current to converge creating high concentration of current levels. Thus, a narrow region of a material will have larger current densities as compared to a wider region of the same material. Accordingly, a “pinch point” in a resistive material will result in a substantial amount of power dissipated as heat, significantly reducing the amount of radiated power. Therefore, for a resistive sheet to be able to radiate power and operate as an antenna, it is as critical to avoid RF current “hot spots” and “pinch points” at both the feeding transitioning area and on the resistive sheet.
A way to address the disadvantages of the efforts attempted by the prior art is to design a feeding mechanism adapted to the topology of the resistive sheet antenna. This would make it possible to increase the radiation efficiency of the overall feeding-antenna system by identifying and mitigating or eliminating the sources of losses experienced both at the feeding transitioning area and as current flows on the resistive sheet. In particular, a feeding topology may be designed to uniformly distribute RF currents on the topology of the resistive sheet that prevent RF current “hot spots” and “pinch points,” resulting in substantial increase of radiation efficiency.
Currently, there is no well-established method of deterministically creating a topology configuration of a feeding mechanism that adapts to the topology of a resistive sheet antenna, to optimize the radiation efficiency of the feeding-antenna system, especially for resistive sheets having a sheet resistivity greater than 10 Ohms per square.
Thus, there remains a need in the art for antenna feeding systems and methods to feed resistive sheet antennas that are capable of operating at radiation efficiencies that avoid the problems of prior art systems and methods.